Infrared Contrasting Color Emissivity Measurement System

ABSTRACT

Devices and corresponding methods can be provided to measure temperature and/or emissivity of a target. Emissivity of the target need not be known or assumed, and any temperature difference between a sensor and the target need not be zeroed or minimized. No particular bandpass filter is required. Devices can include one or two sensors viewing the same target as the target views different respective viewed temperatures. The respective viewed temperatures can be sensor temperatures, and a single sensor can be set to each of the respective viewed temperatures at different times. An analyzer can determine the temperature and/or emissivity of the target based on the respective viewed temperatures and on plural net heat fluxes detected by the sensors and corresponding to the respective viewed temperatures.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 14/190,739, filed Feb. 26, 2014, which claims the benefit of U.S. Provisional Application No. 61/842,169, filed on Jul. 2, 2013. The entire teachings of the above applications are incorporated herein by reference.

BACKGROUND

Non-contact temperature measurement is required for many objects. Conventional infrared radiation sensors determine temperature of a viewed target based on measured heat flux from the target, an emissivity of the target, and the temperature of the sensor. If the target surface has a known emissivity, then the temperature of the target can be accurately measured based on the known emissivity and the output of the radiation sensor. Emissivity can vary from material to material and from surface to surface. Where emissivity of a target is unknown, emissivity can sometimes be assumed, and some techniques exist for decreasing measurement uncertainties due to emissivity assumptions.

SUMMARY

A drawback of conventional temperature measurements involving one sensor temperature is that they often cannot be used to accurately measure target temperature when the emissivity of the target is unknown. Thus, if the emissivity of the target is unknown or changes over time with, for example, a manufacturing process, then a one-temperature sensor measurement may not be sufficient. Furthermore, techniques involving assumed emissivities have drawbacks including increased measurement uncertainty and system cost. Furthermore, although non-contact temperature measurements can be made for targets of unknown emissivity by minimizing or zeroing a temperature difference between the target surface and the sensor, such zeroing of the temperature difference may not always be desirable or possible.

According to the present disclosure, devices and methods are provided for non-contact measurement of targets with sensors, even where emissivity of the target surface is unknown or changes. Furthermore, it is not necessary to assume emissivity or zero any temperature difference between the target and the sensor. Moreover, devices according to the present disclosure can continue to measure temperature even where an emissivity changes, for example, during a manufacturing process.

Devices and methods according to the present disclosure can utilize two sensors held at different temperatures and viewing the same target surface to obtain accurate temperatures and or emissivities based on net heat fluxes measured by the two different sensors held at different temperatures. An alternative method and device according to embodiments of the invention includes using a single sensor to obtain two different heat fluxes corresponding to two different respective temperatures at which the sensor is held at different times. One advantage of these devices and methods is a relative insensitivity to contamination of sensors or sensor windows. In particular, two-sensor devices tend to be insensitive to contamination that is equally distributed on the two sensors. Moreover, one-sensor device can be insensitive to contamination without regard to the degree of contamination of another sensor.

Devices and methods according to the present disclosure can even be used where targets reflect, toward the sensor, radiation from a sensor background surface or sensor housing in addition to radiation from the sensor itself.

A method of determining temperature and corresponding device can include detecting plural net heat fluxes with one or more radiation sensors viewing a target as the target views different respective viewed temperatures. The method can also include determining, independent of an emissivity of the target, a temperature of the target based on the plural net heat fluxes and on the respective viewed temperatures.

The method can include changing a temperature of one of the radiation sensors to achieve the respective viewed temperatures, which can be sensor temperatures. Further, where the temperature of the one radiation sensor is changed, the temperature can be oscillated. The radiation sensors can be thermopile or bolometer sensors, for example.

Determining the target temperature can be performed by an analyzer and can be based further on a transmission factor of a window or filter between the radiation sensor and the target. Determining the target temperature can also be based further on plural sensor-emitted heat fluxes corresponding, respectively, to the respective viewed temperatures. At least one of the net heat fluxes can include heat flux emitted from both a radiation sensor and from an opaque sensor background surface temperature controlled to match a temperature of the radiation sensor. The temperature control can be performed, for example, by a thermal controller.

A method of determining emissivity, and a corresponding device, can include detecting plural net heat fluxes with one or more radiation sensors viewing a target as the target views different respective viewed temperatures. The method can also include determining an emissivity of the target based on the plural net heat fluxes and on the respective viewed temperatures with, for example, an analyzer. Detecting the plural net heat fluxes can be accomplished by using one radiation sensor, and the temperature of the one radiation sensor can be changed to achieve the respective viewed temperatures. Furthermore, the temperature of the one radiation sensor can be oscillated.

The plural net heat fluxes can be detected with respective ones of the radiation sensors. The method can also include calculating a temperature of the target based on the determined emissivity. Determining the emissivity can be performed by an analyzer and can be based further on a transmission factor of a window between the radiation sensor and the target. Determining the emissivity can be based further on plural sensor-emitted heat fluxes corresponding, respectively, to the respective viewed temperatures. The respective viewed temperatures can be sensor temperatures. At least one of the net heat fluxes can include heat flux emitted from both a radiation sensor and from an opaque sensor background surface temperature controlled to match a temperature of the radiation sensor. The one or more radiation sensors can be thermopile or bolometer sensors, for example.

A method of determining temperature can include detecting plural net heat fluxes with a radiation sensor viewing a target as the target views different respective viewed temperatures. The method can also include determining a temperature of the target, independent of an emissivity of the target, based on the plural net heat fluxes and the respective viewed temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.

FIG. 1A is an illustration of two-radiation-sensor device for determining temperature and/or emissivity of a target.

FIG. 1B illustrates radiation emitted from and received by the two radiation sensors in FIG. 1A.

FIG. 2 is a schematic diagram illustrating sources of thermal radiation absorbed and emitted by an infrared sensor viewing an opaque target, using gray body assumptions.

FIG. 3A illustrates a one-radiation-sensor device for determining temperature and/or emissivity of a target.

FIG. 3B illustrates example temperature changes that can be applied to the sensor in FIG. 3A.

FIG. 4A illustrates a radiation sensor as part of a detector assembly.

FIG. 4B illustrates the single-radiation-sensor device of FIG. 3A, further including an opaque sensor background surface temperature controlled to match a temperature of the radiation sensor.

DETAILED DESCRIPTION

One drawback of conventional one-temperature sensor measurements is that their accuracy is limited to the extent that the target emissivity remains uncertain. Further, target emissivity can change over time with, for example, a manufacturing process, and a one-temperature sensor measurement may not be sufficient, even if an initial assumed emissivity is close to the actual value for the target surface.

Furthermore, although emissivity values can be assumed under certain conditions, techniques involving emissivity assumptions remain sensitive to incorrect values of emissivity and have additional drawbacks. For example, a bandpass filter can be used to block longer-wavelength radiation from entering a radiation sensor, making temperature measurements less sensitive to error due to unknown emissivity. However, any improvements offered by this technique are modest, uncertainty can increase due to a lower measurable signal, and a filter specific to the expected target temperature range may be required.

Another technique involves using a narrow bandpass filter to limit radiation incident on a radiation sensor and assuming emissivity is about 1 within the narrow filter bandpass range. The narrow bandpass filter technique is inadequate because radiation sensor signals are small, which sometimes requires that the device be fitted with higher-quality optics and electronics to perform acceptably. This technique can also fail where target emissions are not unusually strong within some bandpass filter range. Furthermore, implementations of this technique are surface-specific and cannot be expected to work satisfactorily for an arbitrary surface.

According to methods described in U.S. Provisional Patent Applications 61/842,204 (filed Jul. 2, 2013) and 61/871,283 (filed Aug. 28, 2013), non-contact temperature measurements can be made for targets of unknown emissivity by minimizing or zeroing a temperature difference between the target surface and the sensor. However, such zeroing of the temperature difference may not be desirable or possible for every application.

According to the present disclosure, devices and methods are provided for non-contact temperature measurement of targets with sensors, even where emissivity of the target surface is unknown or changes. Emissivity need not be assumed, since it can be measured along with temperature. Particular bandpass filters are not required, and radiation sensor signals are adequate such that unusually expensive optical and electrical components are not required. Furthermore, it is not necessary to zero any temperature difference between the target and the sensor.

Devices and methods according to the present disclosure can utilize two sensors held at different respective viewed temperatures and viewing the same target surface to obtain accurate temperatures and or emissivities based on net heat fluxes measured by the two different sensors held at different respective viewed temperatures.

An alternative method and device according to embodiments of the invention includes using a single sensor to obtain two different heat fluxes corresponding to two different respective temperatures at which the sensor is held at different times.

An additional advantage of these devices and methods is a relative insensitivity to contamination of sensors or sensor windows. In particular, two-sensor devices tend to be insensitive to contamination that is equally distributed on the two sensors. Moreover, one-sensor devices can be insensitive to contamination without regard to the degree of contamination of another sensor, and they can also be simpler and less expensive than devices having two or more sensors.

Devices and methods according to the present disclosure can even be used where targets reflect, toward the sensor, radiation from a sensor background surface or sensor housing in addition to radiation from the sensor itself.

FIG. 1A illustrates a device for determining temperature and or emissivity including two radiation sensors 101 a and 101 b. The radiation sensors 101 a-b are configured to view the target 103, which is at an unknown target temperature T_(T) or T_(target). In turn, the target 103 is configured to view the two radiation sensors 101 a-b, which are held at different respective viewed temperatures T_(A) and T_(B), respectively. The radiation sensor 101 a has a field of view 111 a, and the radiation sensor 101 b has a field of view 111 b. In FIG. 1A, the radiation sensors are thermopile sensors. However, in other embodiments, the radiation sensors can be other types of sensors such as bolometers.

A thermistor 105 a monitors the temperature of radiation sensor 101 a and outputs a signal 107 a indicating the viewed temperature T_(A). Similarly, a thermistor 105 b monitors the temperature of the radiation sensor 101 b and outputs a signal 107 b indicating the viewed temperature T_(B). The radiation sensors 101 a and 101 b output signals 109 a and 109 b indicating net heat fluxes Q_(Anet) and Q_(Bnet) detected at the radiation sensors, respectively. While the thermistor 105 a is used to monitor temperature in the device of FIG. 1A, resistance temperature detectors (RTDs) or other types of temperature probes can be used to monitor temperature in other embodiments.

A temperature/emissivity analyzer 121 receives the signals indicating radiation sensor temperatures T_(A) and T_(B) and, since the sensors are themopile sensors, the plural net heat fluxes Q_(Anet) and Q_(Bnet). Based on the measured sensor temperatures T_(A) and T_(B) and the plural net heat fluxes Q_(Anet) and Q_(Bnet), the analyzer 121 can determine the temperature T_(T) of the target 103 using principles that are described hereinafter in conjunction with FIGS. 1B and 2.

The temperature T_(T) can be reported via an output signal 125, and the temperature is determined independent of emissivity of the target. Namely, the emissivity of the target 103 need not be known in order to determine the temperature T_(T). Furthermore, using the same measured sensor temperatures T_(A) and T_(B) and the net heat fluxes Q_(Anet) and Q_(Bnet), the analyzer 121 can determine the emissivity of the target 103, and the emissivity can be reported by the analyzer 121 as an output signal 123. Alternatively, the emissivity can be determined by the analyzer 121 and output without determining the temperature T_(T). The determination of temperature performed by the analyzer 121 is independent of an emissivity of the target because the emissivity of the target can be an arbitrary and unknown value.

FIG. 1B schematically illustrates the radiation emitted from and received at the thermopile radiation sensors 101 a and 101 b. The sensors 101 a and 101 b receive radiation Q_(Atarget) and Q_(Btarget), respectively. In turn, sensors 101 a and 101 b emit radiation Q_(Asensor) and Q_(Bsensor), respectively. The thermopile sensors 101 a and 101 b output signals 109 a and 109 b, which are proportional to Q_(Anet) and Q_(Bnet), respectively. Thus, the net radiation detected by radiation sensor 101 a is

Q _(Anet) =Q _(Atarget) −Q _(Asensor).

Likewise, the net radiation detected at the radiation sensor 101 b is

Q _(Bnet) =Q _(Btarget) −Q _(Bsensor).

The signals 109 a and 109 b, which indicate the Q_(Anet) and Q_(Bnet), respectively, are introduced to the analyzer 121 shown in FIG. 1A for determination of temperature and/or emissivity.

Theoretical considerations useful to determine temperature and emissivity will now be described. For simplicity, the present disclosure assumes that all objects are diffuse emitters. A diffuse emitter is a surface that emits and absorbs radiation from all directions equally well. Diffuse emitters are sometimes termed hemispherical emitters. The results presented herein do not depend on the diffuse emitter assumption, but directionality is nevertheless ignored as a matter of convenience. Note that geometrical view factors are also ignored as a matter of convenience, since the sensor's field of view is typically fixed for this application and the sensor's output signal is a product of the geometrical view factor and the sensitivity of the sensor. Since the output sensitivity of a thermopile is usually individually calibrated for each device as a practical matter, the gain adjustment can be made to account for both the sensitivity of the thermopile and the geometry of the sensor.

A blackbody is a theoretical object that emits thermal radiation at the maximum possible rate at all wavelengths and absorbs all radiation falling upon it. A blackbody functions as the standard to which the emission and absorption of real surfaces are referenced. The energy radiated per unit area by a blackbody at a given temperature and wavelength is given by the Planck Distribution:

E(λ,T)=2πhc ²λ^(−s) /e ^(hc/(kλT)−1   (1.1)

where h is Planck's constant (6.6262×10 ⁻³⁴ J s), c is the speed of light (2.9979246×10⁸ m/s), λ is the wavelength of the radiation, k is the Boltzmann constant (1.380662×10²³J/K), and T is the temperature of the surface in Kelvin.

Emissivity, ε, is a property of a physical surface, and it refers to the efficiency of emitting and absorbing infrared radiation, as a fraction of the radiation that would be emitted or absorbed by an equivalent blackbody. Therefore, the values of emissivity for real surfaces vary between 0 and 1. Emissivity can vary as a function of wavelength, temperature, and even with time as the condition of a surface changes.

The total amount of energy emitted by a real object (non-blackbody) can be found by integrating the product of the emissivity of the object and the energy emitted by a blackbody, Equation 1.1 over all wavelengths:

Q=∫₀ ^(∞)ε(λ,T)E(λ,T)dλ   (1.2)

An assumption often made is that emissivity is independent of wavelength (and everything else); the surface is then referred to as being a “greybody.” In the case of a greybody, the emissivity is a constant between zero and one, and evaluating the above integral gives:

Q=εσT⁴   (1.3)

where σ is the Stefan-Boltzmann constant.

FIG. 2 is a schematic diagram illustrating sources of thermal radiation absorbed and emitted by an infrared sensor 201 using greybody assumptions. The IR sensor 201 emits energy 251. An opaque target 203 emits energy 253. When infrared energy 257 from a foreground is incident on the surface of the target 203, a portion 255 of the foreground energy is reflected.

As illustrated in FIG. 2, the net heat flow at the sensor is a combination of energy that is reflected and emitted by the target surface, minus the energy that is emitted by the sensor itself. This net heat flow actually measured by the thermopile is termed Q_(net) in Equation 1.4:

Q _(net) =−Q _(sensor) +Q _(emitted) +Q _(reflected)   (1.4)

Any given radiation that falls on a surface is either absorbed, reflected, or transmitted through the object. Under steady state conditions, absorption is equal to emissivity, and:

ε+τ+r=1

where τ is the transmissivity, and r is the reflectivity of the target. Further, where the target is opaque, τ=0, and the relationship becomes simply:

ε+r=1   (1.5)

In order to use the output signal of a thermopile to compute the temperature of the target object being measured, an independent reference measurement of the temperature of the thermopile is needed. This reference temperature is required to compute the value of the heat flux emitted by the absorber area of the thermopile, Q_(sensor) in Equation 1.4.

Radiation reflected from the target and received by the sensor can include background and foreground sources, in addition to radiation emitted by the sensor. Background sources include sources in the vicinity of the sensor, such as a sensor housing or sensor background surface, which can be viewed by the target surface. Foreground sources include sources other than the target surface that can be viewed by the sensor or otherwise detected by the sensor after reflection from the target surface. These sources can include ambient air and objects other than the sensor and target that emit radiation that can leak into a space between the sensor and target, for example.

In the case where the target is a greybody, and where all sources of reflections from the target are at the same temperature as the sensor, the background, foreground, and sensor behave as blackbodies; and the sensitivity of the thermopile is invariant with respect to wavelength. Under this greybody condition, and starting from Equations 1.3 and 1.4, the temperature of the target can be computed as:

Q _(net) =−σT ⁴ _(sensor) +εσT ⁴ _(target) +rσT ⁴ _(sensor)

Q _(net)=−(1-r)σT ⁴ _(sensor) +εσT ⁴ _(target)

Taking note of Equation 1.5, this can be simplified to:

Q _(net) =εσT ⁴ _(target) −εσT ⁴ _(sensor)   (1.6)

Equation 1.6 can also be written as

Q _(net)=εσ(T _(T) ⁴ −T _(S) ⁴),

where T_(T) is T_(target) and T_(S) is T_(sensor).

Rearranging Equation 1.6 to Solve for the Temperature of the Target Gives:

$\begin{matrix} {T_{tartget} = \sqrt[4]{\frac{Q_{net}}{ɛ\sigma} + T_{sensor}^{4}}} & (1.7) \end{matrix}$

In the real world, it is rare for all of those assumptions to be true. For example, since sensors built for measuring the IR radiation from cool targets normally have a filter window installed whose transmission of IR varies with wavelength, the integration of Equation 1.2 requires an additional term in the integrand for the variable transmission with wavelength, and becomes:

Q=∫₀ ^(∞τ) _(filter)(λ)ε(λ,T)E(λ,T)dλ   (1.8)

If the target to be measured is assumed to be a greybody, then the emissivity in Equation 1.8 is constant and can be pulled out of the integrand, and the equation simplifies to:

Q=ε∫ ₀ ²⁸τ_(filter)(λ)E(λ,T)dλ   (1.9)

Two assumptions can then be made with appropriate construction of the system, namely that the emissivity of the sensor's active area is 1, and the ambient background temperature surrounding the sensor is uniform and identical to the temperature of the sensor. With these assumptions, Equation 1.4, Equation 1.5, and Equation 1.9 can be combined and give:

$\begin{matrix} {{{\int_{0}^{\infty}{{\tau_{filter}(\lambda)}{E\left( {\lambda,T_{target}} \right)}d\; \lambda}} = {\frac{Q_{net}}{ɛ} + {\int_{0}^{\infty}{{\tau_{Filter}(\lambda)}{E\left( {\lambda,T_{sensor}} \right)}d\; \lambda}}}},} & (1.10) \end{matrix}$

where T_(sensor) is the viewed temperature of the sensor viewed by the target.

The integrations of Equation 1.10 can be carried out numerically, and the results can be used to generate a function f, wherein the target temperature is given as a function of the other variables involved:

$\begin{matrix} {T_{target} = {f\left( {\frac{Q_{net}}{ɛ} + {\int_{0}^{\infty}{{\tau_{filter}(\lambda)}{E\left( {\lambda,T_{sensor}} \right)}d\; \lambda}}} \right)}} & (1.11) \end{matrix}$

If ε is known, Equation 1.11 can be used to calculate the target temperature. Moreover, where E is not known or changes over time, E can be determined using two identical sensors such as 101 a-b viewing the same target, but held at different sensor temperatures. Using two sensors at different respective viewed sensor temperatures, it is possible to solve for both target temperature and the emissivity of the surface being measured.

Each sensor 101 a-b in the pair is governed by Equation 1.10. If the sensors are identical in construction and are looking at the same target surface at the same time, all of the terms in the integral on the left side of Equation 1.10, (τ_(filter) and E(λ, T_(target))), are identical, therefore the entire integral is identical in the equation for each sensor. The equality for the right hand side of the two versions of Equation 1.10 for the pair of sensors 101 a-b can be written as:

$\begin{matrix} {{{\frac{Q_{Anet}}{ɛ} + {\int_{0}^{\infty}{{\tau_{filter}(\lambda)}{E\left( {\lambda,T_{target}} \right)}d\; \lambda}}} = {\frac{Q_{Bnet}}{ɛ} + {\int_{0}^{\infty}{{\tau_{filter}(\lambda)}{E\left( {\lambda,T_{Bsensor}} \right)}d\; \lambda}}}},} & (1.15) \end{matrix}$

where T_(Asensor) and T_(Bsensor) are different respective viewed temperatures viewed by the target surface.

Using the term Q_(sensor) for the heat flux emitted by the sensor (the physical meaning of the remaining integrals), solving Equation 1.15 for emissivity, gives:

$\begin{matrix} {ɛ = \frac{\left( {Q_{Anet} - Q_{Bnet}} \right)}{\left( {Q_{Asensor} - Q_{Bsensor}} \right)}} & (1.16) \end{matrix}$

It can be seen from Equation 1.16 that for a meaningful calculation of emissivity, the sensors must be at different temperatures. The value of ε calculated from Equation 1.16 can be used in Equation 1.11 to calculate a temperature that is independent of emissivity of the target, i.e., can be calculated regardless of the target emissivity and without prior knowledge of the value of the target emissivity. Further, the temperature may be determined directly if desired, without a separate step of determining emissivity. Specifically, using two versions of Equation 1.10, one for each sensor, ε falls out, and the equations can be solved for target temperature as follows:

$\begin{matrix} {{T_{target} = {f\left( \frac{{Q_{Anet}{\int_{0}^{\infty}{{\tau_{filter}(\lambda)}{E\left( {\lambda,T_{Bsensor}} \right)}d\; \lambda}}} - {Q_{Bnet}{\int_{0}^{\infty}{{\tau_{filter}(\lambda)}{E\left( {\lambda,T_{Asensor}} \right)}d\; \lambda}}}}{\left( {Q_{Anet} - Q_{Bnet}} \right)} \right)}},} & (1.17) \end{matrix}$

where the left and right integrals can be termed Q_(Bsensor) and Q_(Asensor), respectively, as done in Equation 1.16.

For most applications, the emissivity can be expected to vary far more slowly than the target temperature. In those applications, a low pass filter with a fairly low frequency cut-off (on the order of seconds, and perhaps even on the order of minutes) can be applied to the determined emissivity. If such a filter is used, any errors arising from the determined c can be further reduced by determining the target temperature using the data from the sensor of the pair that minimizes the quantity |T_(target)-T_(sensor)|.

Moreover, the above-described method for determining emissivity and/or temperature can also be performed using a device with only one radiation sensor. In most applications requiring the measurement of the temperature of an object with unknown and/or variable emissivity, emissivity is a constant, or emissivity slowly varies due to changes to the surface of the target. In applications where emissivity can be treated as constant over moderate periods of time, the advantages of viewing a target with two sensors held at different temperatures can be achieved using a single sensor by changing a temperature of the sensor with time. Sensor readings separated in time can then be used in the same way, as described above for separate sensors, to determine emissivity of the target surface. Moreover, multiple sensor measurements of heat flux can be obtained at each temperature by causing the temperature of the sensor to oscillate in time. Using a single sensor design can have additional advantages compared with a two-sensor design, such as decreased power draw, decreased need for large and uniform target surfaces, decreased cost, and decreased sensitivity to calibration precision of sensors.

FIG. 3A shows a single-sensor device for determining temperature and/or emissivity of the target surface 103. A particular advantage of the single-sensor device of FIG. 3A is insensitivity to greybody contamination of the sensor window (not shown), without regard to any degree of contamination of any second sensor window. In FIG. 3A, a single radiation sensor 301 is configured to view the target surface 103 with a field of view 311. A thermistor 305 monitors a temperature of the sensor 301, and a heater 327 controls the temperature of the radiation sensor 301. The heater 327 is controlled by a thermal controller, which is not shown. In other devices, both heating and cooling can be required for thermal control of the sensor, and thermoelectric coolers, for example, can be used.

The heater 327 causes the radiation sensor 301 to oscillate in temperature between two different respective viewed temperatures T_(A) and T_(B). The temperatures T_(A) and T_(B) are monitored by the thermistor 305 and reported in the form of an output signal 307. The radiation sensor 301 measures the plural net heat fluxes Q_(Anet) and Q_(Bnet), which are reported in the form of an output signal 309. With the two temperatures T_(A) and T_(B) and the two net heat fluxes Q_(Anet) and Q_(Bnet) measured, the emissivity and or temperature of the target 103 can be determined using the same equations as described above in conjunction with FIGS. 1A-1B and 2. Specifically,

$\begin{matrix} {{ɛ = \frac{\left( {Q_{Anet} - Q_{Bnet}} \right)}{\left( {Q_{Asensor} - Q_{Bsensor}} \right)}}{and}} & (1.16) \\ {T_{target} = {{f\left( \frac{{Q_{Anet}{\int_{0}^{\infty}{{\tau_{filter}(\lambda)}{E\left( {\lambda,T_{Bsensor}} \right)}d\; \lambda}}} - {Q_{Bnet}{\int_{0}^{\infty}{{\tau_{filter}(\lambda)}{E\left( {\lambda,T_{Asensor}} \right)}d\; \lambda}}}}{\left( {Q_{Anet} - Q_{Bnet}} \right)} \right)}.}} & (1.17) \end{matrix}$

FIG. 3B shows the temperature of the radiation sensor 301 as a function of time. The temperature is initially ramped up to the higher temperature T_(B), and the temperature is allowed to stabilize over a time period 332. At the end of the time period 332, Q_(Bnet) is measured by the radiation sensor 301 at time 334. Following the measurement of Q_(Bnet), at 331 the temperature is ramped down to the lower temperature T_(A). Similarly, the temperature T_(A) is allowed to stabilize over a period of time, and following the stabilization period, Q_(Anet) is measured at time 336. The emissivity and temperature of the target 103 are then determined based on the measurements, and the process is repeated as the temperature of the radiation sensor 301 continues to oscillate. For some applications, instead of determining a target temperature each time Q_(Anet) and Q_(Bnet) are measured, the target temperature is determined only after measuring Q_(Anet) and Q_(Bnet) multiple times over several temperature cycles and averaging. Moreover, in some applications, the temperature profile can be different from that shown in FIG. 3B.

The temperature output of the device in FIG. 3A can output temperatures at the fastest sampling rate possible, and a digital filter can be applied to the inputs for the emissivity and temperature calculations. The device can then be enabled to accurately track fast transients in temperature, and reporting of changes in emissivity can be limited to those that occur at frequencies lower than twice the rate at which the sensors temperature is oscillated, for example. Furthermore, any errors due to aliasing of changes in the target temperature to the device temperature can be minimized by oscillating the temperature of the sensor 301 in a stochastic manner. Namely, instead of oscillating the sensor's temperature at a fixed frequency, the frequency and magnitude of the temperature variation can be varied randomly or pseudorandomly. The stochastic approach can limit the maximum possible errors generated by coincident oscillation in the target temperature. Limiting these errors can be important because many temperature control systems use an output with a regular frequency, and control loops often have a small oscillation about the setpoint, termed “hunting” when they are in operation.

FIGS. 4A and 4B illustrate that radiation reflected from a target surface can include radiation emitted from sources other than a sensor. FIG. 4A illustrates how the sensor 301 can be part of a detector assembly 429. The detector assembly 429 is maintained at the same temperature as the sensor 301 by the heater 327. Thermal radiation detected at the sensor 301 includes radiation emitted from the detector assembly 429 and reflected from the target 103. Because the detector assembly 429 is maintained at the same temperature as the sensor 301, any contribution of radiation from the detector assembly 429 to the net heat flux measured by the radiation sensor 301 does not affect the equations previously described that allow temperature and emissivity to be determined.

FIG. 4B illustrates that the device of FIG. 3A can also be used where the sensor 301 detects net heat fluxes that include radiation emitted from other surfaces. In FIG. 4B, a heater 427 controls the temperature of a sensor background surface 431. Provided the temperature of the sensor background surface 431 is maintained at the same temperature as the radiation sensor 301, radiation emitted from the sensor background surface 431 and reflected from the target surface 103 to contribute to the net heat fluxes Q_(Anet) and Q_(Bnet) does not impact the device's ability to determine temperature and emissivity of the target surface 103 as previously described.

The temperature-controlled detector assembly 429 (in FIG. 4A) and the sensor background surface 431 (in FIG. 4B) can be used to exclude or limit any radiation from sources that are not at the same temperature as the sensor from being detected by the sensor. Thus, for example, even if the temperature of the ambient area around the sensor is not at the sensor temperature, sensor temperature and emissivity can still be measured accurately provided that the detector assembly 429 or background surface 431 is sufficiently large or sufficiently close to the target surface 103 to effectively block radiation originating from sources external to the device.

The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 

What is claimed is:
 1. A method of determining emissivity, the method comprising: detecting plural net heat fluxes with one or more radiation sensors viewing a target as the target views different respective viewed temperatures; and determining an emissivity of the target based on the plural net heat fluxes and on the different respective viewed temperatures.
 2. The method of claim 1, further comprising changing a temperature of one of the radiation sensors to achieve the respective viewed temperatures.
 3. The method of claim 2, wherein changing the temperature of the one radiation sensor comprises oscillating the temperature of the one radiation sensor.
 4. The method of claim 1, wherein detecting the plural net heat fluxes is with respective ones of the radiation sensors.
 5. The method of claim 1, further comprising calculating a temperature of the target based on the determined emissivity.
 6. The method of claim 1, wherein determining the emissivity is based further on a transmission factor of a window between the radiation sensor and the target.
 7. The method of claim 1, wherein determining the target temperature is based further on plural sensor-emitted heat fluxes corresponding, respectively, to the respective viewed temperatures.
 8. The method of claim 1, wherein the respective viewed temperatures are sensor temperatures.
 9. The method of claim 1, wherein at least one of the net heat fluxes includes heat flux emitted from both a radiation sensor and from and opaque sensor background surface temperature controlled to match a temperature of the radiation sensor.
 10. The method of claim 1, wherein the one or more radiation sensors are thermopile or bolometer sensors.
 11. An emissivity measurement device comprising: one or more radiation sensor configured to view a target and to detect plural net heat fluxes as a target views different respective viewed temperatures; and an analyzer configured to determine an emissivity of the target based on the plural net heat fluxes and on the respective viewed temperatures.
 12. The device of claim 11, further comprising thermal controller configured to change a temperature of one of the radiation sensors to achieve the respective viewed temperatures.
 13. The device of claim 12, wherein the thermal controller is further configured to cause the temperature of the one radiation sensor to oscillate.
 14. The device of claim 11, further comprising two or more radiation sensors configured to detect respective ones of the plural net heat fluxes.
 15. The device of claim 11, wherein the analyzer is further configured to determine a temperature of the target based on the determined emissivity.
 16. The device of claim 11, wherein the analyzer is further configured to determine the emissivity of the target based further on a transmission factor of a window between the radiation sensor and the target.
 17. The device of claim 11, wherein the analyzer is further configured to determine the emissivity of the target based further on plural sensor-emitted heat fluxes corresponding, respectively, to the respective viewed temperatures.
 18. The device of claim 11, wherein the respective viewed temperatures are sensor temperatures.
 19. The device of claim 11, wherein at least one of the plural net heat fluxes includes heat flux emitted from both a radiation sensor and from an opaque sensor background surface temperature controlled to match a temperature of the radiation sensor.
 20. The device of claim 11, wherein the radiation sensors are thermopile or bolometer sensors. 